This invention relates generally to the cooling of operating devices on orbiting spacecraft and more particularly to heat pipe assemblies constructed of different materials that optimize heat transfer conditions independent of thermal expansion constraints. However, the invention is also applicable to terrestrial applications.
The environmental conditions encountered by an orbiting spacecraft carrying various devices whose operation generates unwanted heat require that they be cooled if the mission is to be successful. It is well known to cool the operating devices using heat pipe assemblies that must function in a space vehicle environment that demands the use of compatible materials, light in weight and provide optimal results. Conditions are further complicated due to extreme temperature changes such as when as the spacecraft passes through the cold eclipse of its orbit and the much higher temperatures as it sees the sun's radiation. The construction and operation of heat pipes is well known such as described in U.S. Pat. No. 4,170,262 issued Oct. 9, 1979, and assigned to the same assignee as the instant invention and is incorporated herein by reference.
Effective heat removal from the operating device depends on using heat pipes such as the type described in U.S. Pat. No. 4,170,262, assigned to the same assignee as this invention, depends on transfer of heat by conduction to the heat pipe which then convects the removed heat to radiating panels that radiate the heat into ambient space. Successful operation of space missions therefore, particularly communication satellites, require cooling the various on-board devices. Good spacecraft design requires the use of lightweight materials for the construction of the apparatus essential for its mission and any ancillary equipment must be within the controlling weight limits. It is also desirable to provide conditions that optimize heat transfer by conduction and at the same time meet the preferred requirements of satellite construction calling for lightweight materials that lend themselves to easy assembly and fabrication and can survive frequent cycling over extreme temperature ranges from -50.degree. F. to 200.degree. F.
In practice the heat-producing operating device will have affixed to, or within its structure, a heat pipe assembly comprising a heat absorbing host structure equipped with radiating panels having embedded therein a heat pipe such as the type disclosed in the aforementioned U.S. Patent. As the heat from the operating device is conducted to the host structure, it vaporizes the working fluid in the heat pipe which is then condensed and the heat of condensation conducted to the radiating panels which dissipate it into the ambient space. The embedded heat pipe takes heat out of the operating device at its evaporator end and the vapors are condensed at its condenser end giving up heat to the host structure and discharging it to ambient space.
Currently the heat pipe assemblies are fabricated using the same materials for the host structure and the heat pipes. Such a construction strategy avoids any problem caused by components having different coefficients of thermal expansion (CTE) that could cause stress failure in the assembly. Limiting material selection is a compromise that affects the efficiency performance of the heat pipe, contributes to the weight of the satellite and is less than optimum in terms of heat removal. Recent advances in materials science that could improve the construction of heat pipe assemblies have not been realized due to the thermal stress imposed by the differential CTE between the host structure and the firmly affixed heat pipe. Thermally-induced stress causes detachment or debonding of the heat pipe from host structure and therefore failure of the thermal control system.